The medial olivocochlear (MOC) efferent system is known to exert significant effects on cochlear physiological activity, yet little is known about the function of the efferents in auditory perception or the behavioral response of hearing. The proposed functions of the MOC, relevant to the present work, include: 1) to unmask or decompress an auditory nerve fiber's I/O function by suppression of the neural representation of concurrent noise; 2) to increase the salience of transient signal onset by mediating the process of neural adaptation, which acts to decrease or suppress the representation of long-duration stimulation; and 3) to provide a peripheral, neural mechanism that underlies the process of "selective" auditory attention, by which the neural encoding of "unattended" stimuli are suppressed. This application proposes a unique experimental approach to characterizing the role of the efferent tracts in auditory perception in humans, combining psychophysical and physiological (DPOAE) measures in the same subjects. We propose to develop psychophysical tasks as analogs of known efferent physiological processes, focusing on psychophysical measures related to the above, purported efferent processes. Specific psychophysical tests will employ both human and non-human primate subjects as listeners, in parallel, under identical stimulus conditions. The DPOAE measures will serve to aid in specification of the precise stimulus conditions to be tested, being optimized for specific aims. It is assumed, based on previous psychophysical and anatomical studies, that the humans and monkeys will yield comparable data under similar stimulus conditions. The ultimate goal of this research program is characterization of the contribution of the medial efferent tracts to auditory perception in humans. Parallel testing of human and non-human subjects permits unequivocal determination of the contribution of the efferents to the different perceptual processes studied, in both monkeys and humans, through lesion studies in the monkey subjects. Without this parallel testing paradigm (and the ability to lesion tracts in the non-human primate subjects) the link between human perception and the underlying neural mechanism would not be direction obtainable.